Miller-Urey experiment

The Miller–Urey experiment was a chemical experiment that simulated the conditions thought at the time (1952) to be present on the early Earth and tested the chemical origin of life under those conditions. After Miller’s death in 2007, scientists examining sealed vials preserved from the original experiments were able to show that there were actually well over 20 different amino acids produced in Miller’s original experiments. That is considerably more than what Miller originally reported, and more than the 20 that naturally occur in the genetic code. More recent evidence suggests that Earth’s original atmosphere might have had a composition different from the gas used in the Miller experiment…(Source: Wikipedia)

Atmosphere of early Earth had oxygen